1. Field of the Invention
The present invention relates to a mold material and a process for stably casting pure titanium or a titanium alloy whose properties are unstable in a molten state.
2. Description of the Prior Art
Up to this time, a mold material comprising silica as a chief material and a phosphate and a basic metal oxide as a hardening agent has been used for casting various metals. Quartzite, quartz sand or the like has been used as such silica and ammonium phosphate or the like has been used as such a phosphate, while electromelting magnesia, magnesia clinker or the like has been used as such a basic metal oxide.
The casting of a metal by the use of the above mold material of the prior art is generally carried out by firing a mixture of the chief material and the hardening agent at 800.degree. to 900.degree. C. to prepare a mold and casting a molten metal into the mold kept at this temperature.
Most of metals of the prior art have a melting point of up to about 1400.degree. C. Representatively, cobalt-chromium alloy, nickel-chromium alloy or the like has a melting point of 1300.degree. to 1400.degree. C. Particularly, a dental metal has a melting point of up to about 1400.degree. C. Accordingly, the above-described mold material containing silica having a melting point of 1550.degree. to 1600.degree. C. as a chief material can be sufficiently used for casting these metals. However, pure titanium or a titanium alloy has a melting point of 1600.degree. to 1700.degree. C., which exceeds the melting point of the above mold material containing silica as a chief material. Accordingly, when titanium which is unstable in a molten state is cast by the use of the above mold material, the pure titanium or titanium alloy is bonded to oxygen contained in the silica to form titanium oxide, so that the obtained product does not exhibit the inherent characteristics of pure titanium or titanium alloy and is not fit for use because of the deterioration of the metal due to oxidation, the seizing of the surface of the product and the like.